NEC Section 110.26 has 3 requirements for units with nominal voltage to ground between 151 and 600V:
Condition 1 – Exposed live parts on one side of the working space and no live parts or grounded parts on the other side of the working space or exposed live parts on both sides of the working space that are effectively guarded by insulating materials. 3 ft of working space.
Condition 2 – Exposed live parts on one side of the working space and grounded parts on the other side of the working space. Concrete, brick, or tile walls shall be considered grounded. 3 ft of working space.
Condition 3 – Exposed live parts on both sides of the working space. 4 ft of working space.
The IOM calls out 3 ft as shown below with EH.